1. Field of the Invention
The present invention relates to a liquid crystal display device for use in a direct-view display, a projection display and the like.
2. Description of the Related Art
In recent years, the size and weight of office automation apparatuses such as a personal computer have been reduced, and portable information apparatuses which are usually carried by persons have been put into a practical use. As a display unit for such information apparatuses, a liquid crystal display (LCD) device using liquid crystal has been used most widely because of low power consumption, small size and light weight. As such a liquid crystal display device, a transmission type liquid crystal display device using a back light is usually utilized. In recent years, low power consumption displays have been required and a reflection type liquid crystal display device using no back light has been developed vigorously.
As an example of conventional reflection type liquid crystal display device, a reflection type liquid crystal display device disclosed in Japanese Unexamined Patent Publication JP-A 5-323371 (1993) will be described below.
FIG. 15 is a plan view showing a conventional reflection type liquid crystal display device, and FIG. 16 is a sectional view taken along line Axe2x80x94A of the LCD device shown in FIG. 15. A plurality of scanning lines 102 formed of a material such as aluminum, tantalum or the like are disposed in parallel with each other on an insulating substrate 101 made of glass or the like. A gate electrode 103 branches off from each of the scanning lines 102. A gate insulating film 104 made of nitride silicon (SiNx), silicon oxide (SiO2) or the like is disposed over the whole substrate 101 to cover the gate electrode 103. A semiconductor active layer 105 made of amorphous silicon, polycrystalline silicon or the like is disposed on the gate insulating film 104 provided on the gate electrode 103. A contact electrode 106 made of amorphous silicon to which impurity ions are added, microcrystal silicon, polycrystalline silicon or the like is disposed on both ends of the semiconductor active layer 105. A source electrode 107 and a drain electrode 108 which are made of aluminum, titanium, tantalum, chromium or the like are disposed on the contact electrodes 106 formed on the both ends.
As shown in FIG. 16, a signal line 110 intersecting with the scanning line 102 with the gate insulating film 104 interposed therebetween is connected to the source electrode 107. The signal line 110 is also formed of the same material as the material of the source electrode 7. The gate electrode 103, the gate insulating film 104, the semiconductor active layer 105, the contact electrode 106, the source electrode 107 and the drain electrode 108 constitute a thin film transistor (TFT) 111. The TFT 111 has the function of a switching element.
An interlayer insulating film 112 comprising an inorganic material such as nitride silicon or an organic material is formed over the whole substrate 101 to cover the scanning line 102, the signal line 110 and the TFT 111. A pixel electrode 113 comprising a material having a high reflectivity such as Al is formed on the interlayer insulating film 112. A contact hole 114 is formed in a portion of the interlayer insulating film 112 which overlaps with the drain electrode 108. The pixel electrode 113 and the drain electrode 108 are connected to each other through the contact hole 114. Furthermore, an orientation film (not shown) is formed on the pixel electrode 113. Thus, an active matrix substrate portion is formed.
An opposite substrate portion is disposed to be opposite to the active matrix substrate portion. In the opposite substrate portion, a color filter 116 is formed on an insulating substrate 115 made of glass or the like. One of red, green and blue color layers 116R, 116G and 116B is formed in a region of the color filter 116 corresponding to the pixel electrode 113, and a metallic shielding film (black matrix) 116BM made of chromium nitride, tantalum nitride or the like is disposed in a region opposite to a region between the pixel electrodes 113 or to the signal line 110. The black matrix is formed of black resin and the like other than metals. A common electrode 117 comprising a transparent conductive material such as ITO is formed on the color filter 116. A liquid crystal layer 118 is provided between the active matrix substrate portion and the opposite substrate portion.
Next, the operation of a reflection type liquid crystal display device having such a structure will be described. When the TFT 111 is turned on, a current flows from the signal line 110 to the pixel electrode 113 and the pixel electrode 113 is charged to the voltage of the signal line 110 obtained at that time. At this time, a voltage is applied to the liquid crystal layer 118 interposed between the pixel electrode 113 and the common electrode 117 so that the liquid crystal layer 118 operates. In the reflection type liquid crystal display device, light incident from the opposite substrate portion side is reflected by the pixel electrode 13, thereby performing display. The light incident from the opposite substrate portion side is reflected by the pixel electrode 113 and polarized by the liquid crystal layer 118 so that the transmittances of pixels differ from each other. Consequently, a contrast is formed between two or more pixel electrodes 113 and image display is accomplished.
On the insulating substrate 115 of the opposite substrate portion is formed the color filter 116, in which the black matrix 116 BM is formed in a region of the insulating substrate to be opposite to a region between the pixel electrodes 113 or to the signal line 110. In order to reduce the cost of the color filter, a structure free from the black matrix 116 BM (which will be hereinafter referred to as a BM-less structure) has been examined. This is because there is a problem that the manufacturing cost is greatly increased in the case of use of a metal film for the black matrix. A possible BM-less structure will be described with reference to FIG. 17.
(1) As shown in FIG. 17A, no black matrix is formed in a region of the insulating substrate 115, opposite to a region between the pixel electrodes 113 or to the signal line 110, and the common electrode 117 and the orientation film 151 are directly disposed on the insulating substrate 115.
(2) As shown in FIG. 17B, in the region opposite to the region between the pixel electrodes 113 or to the signal line 110 are overlapped the red color layer 116R and the green color layer 116G, on which the common electrode 117 and the orientation film 151 are laminated.
Alternatively, the red color layer 116R and the green color layer 116G may be disposed to be adjacent leaving no interval therebetween in the region opposite to the region between the pixel electrodes or to the signal line and the scanning line, instead of overlapping the color layers of the color filter. By overlapping the color layers of the color filter or arranging the color layers to be closely adjacent, light can be more shielded than in the examples of FIG. 17A and FIG. 17C, which will be described below.
(3) As shown in FIG. 17C, an insulating film 152 is formed on the color filter 116 to be flat. In that case, the insulating film 152 is embedded in the region opposite to the region between the pixel electrodes 113 or to the signal line 110. The common electrode 117 and the orientation film 151 are provided on the insulating film 152.
With the structures shown in FIGS. 17A to 17C, shielding properties are more deteriorated than in the structure having the black matrix. Therefore, light transmission occurs in the region opposite to the region between the pixel electrodes 113 or to the signal line 110. In the case where the prior art reflection type liquid crystal display device has the BM-less structure as shown in FIG. 18, light 121 incident from the surface of the reflection type liquid crystal display device hits upon the signal line 110 of an interval 119 between the pixel electrodes 113 (which is equivalent to the black matrix 116 BM), thereby causing surface reflection because a metal having a high reflectivity such as aluminum, titanium or the like is used for a line material. This phenomenon causes the following drawbacks. Since a fluctuating voltage is always applied to the signal line 110, a voltage corresponding to a voltage applied to a liquid crystal of a region of the LCD device where gradation display is performed is always applied to the liquid crystal layer 118 provided above the signal line 110. For this reason, the liquid crystal layer 118 provided above the signal line 110 has light transmitting properties corresponding to the voltage. Accordingly, in the case where the incident light is reflected by the signal line 110 and black display is performed in the pixel electrode 113, the contrast decreases due to the reflected light.
Moreover, a voltage which is equal to or higher than a threshold voltage of the liquid crystal is applied onto the scanning line 102. The liquid crystal layer 118 provided on the scanning line 102 has a transmittance of 100% when the liquid crystal is set in a normally black mode. Therefore, the same problem arises.
Japanese Examined Patent Publication JP-B-2 8-12353 (1996) discloses a liquid crystal display device in which a gate line, that is, a scanning line is formed of a conductive material for shielding light, and a shielding film comprising a material identical to the material of the gate line is formed along a source line, that is, a signal line apart from the source line. Also in the liquid crystal display device having such a structure, a shielding film should be provided in a region of a color filter which is opposite to the line in order to increase the effect of shielding light.
It is an object of the invention to provide a liquid crystal display device having a structure in which light reflection of a line can be suppressed and a decrease in contrast can be prevented without providing a shielding film in an opposite substrate portion.
The invention provides a liquid crystal display device comprising a liquid crystal layer, an active matrix substrate portion and an opposite substrate portion which is provided opposite to the active matrix substrate with the liquid crystal layer interposed therebetween,
the active matrix substrate portion having:
a first insulating substrate;
a plurality of scanning lines provided on the first insulating substrate;
a plurality of signal lines provided to intersect with the scanning lines through an insulating film;
a plurality of switching elements provided in the vicinity of the intersecting portions of the scanning lines with the signal lines; and
a plurality of pixel electrodes connected to the switching elements, respectively,
the signal lines being provided in a region between the pixel electrodes on the first substrate, and
the opposite substrate portion having:
a second insulating substrate;
a color filter provided on the second substrate; and
a common electrode provided on the second substrate, comprising a transparent conductive material,
wherein color layers of the color filter are formed in a region opposite to the pixel electrode, and a light transmitting portion is provided in a region opposite to at least a part of the region between the pixel electrodes, and
number of components of light which can be transmitted through a light transmitting portion is large than that of light which can be transmitted through the color layers, and the signal lines are formed of a transparent conductive material.
According to the liquid crystal display device of the invention, the light incident from the opposite substrate portion side of the liquid crystal display device is reduced to be reflected by the signal lines and is transmitted through the insulating film and the substrate of the active matrix substrate portion to the back face of the liquid crystal display device. Therefore, when black display is performed, surface reflection is reduced. Consequently, a black level can be improved and a contrast can be enhanced.
In the invention it is preferable that the pixel electrode has a reflecting function.
According to the invention, also in the case of a reflection type liquid crystal display device in which the pixel electrode has the reflecting function, the light incident from the opposite substrate portion side of the reflection type liquid crystal display device is reduced to be reflected by the signal lines and is transmitted through the insulating film and the substrate of the active matrix substrate portion to the back face of the liquid crystal display device. Therefore, when black display is performed, surface reflection is reduced. Consequently, a black level can be improved and a contrast can be enhanced. Thus, in the case where at least a portion of the pixel electrode has the reflecting function, the above-mentioned effects can be particularly produced remarkably by the structure in which the signal lines transmit light.
In the invention it is preferable that reflectivity R of the signal line and ratio (AS/AD) of area AS of the signal line to area AD of the pixel electrode ranges within a region enclosed by a straight line A defined by expressions (1), (2), a straight line B defined by expressions (3), (4) and a curve C defined by an expression (5).
Rxe2x89xa7Rminxe2x80x83xe2x80x83(1)
                    R        ⁢                  xe2x80x83                ⁢                  min          ⁡                      (                                                            (                                                            n1                      -                      n2                                                              n1                      +                      n2                                                        )                                2                            +                                                (                                      1                    -                                                                  (                                                                              n1                            -                            n2                                                                                n1                            +                            n2                                                                          )                                            2                                                        )                                xc3x97                                                      (                                                                  n2                        -                        n3                                                                    n2                        +                        n3                                                              )                                    2                                                      )                          xc3x97        100                            (        2        )            
n1: refractive index of an upper layer portion which is in contact with the signal line
n2: refractive index of the signal line
n3: refractive index of a lower layer portion which is in contact with the signal lines
(AS/AD)xe2x89xa7Bminxe2x80x83xe2x80x83(3)
                              B          ⁢                      xe2x80x83                    ⁢          min                =                              (                          f              xc3x97              h1                        )                                              (                              f                -                h2                -                                  2                  ⁢                  i                                            )                        xc3x97                          (                              g                -                h1                -                                  2                  ⁢                  i                                            )                                                          (        4        )            
f: longitudinal length of one pixel
g: transverse length of one pixel
h1: signal line width
h2: scanning line width
i: sampling width
(AS/AD)xe2x89xa7(YWxe2x88x9215YB)/(14YMxc3x97R)xe2x80x83xe2x80x83(5)
YW: brightness obtained in white display state
YB: brightness obtained in black display state
VW: voltage to be applied in white display state
VB: voltage to be applied in black display state
YM: brightness obtained with an applied voltage of (VW+VB)/2
According to the invention, moreover, the area of the pixel electrode, the area of the signal line and the reflectivity on the surface of the signal line are optimized. Consequently, a contrast can be enhanced in a liquid crystal display device having any specification.
In the invention it is preferable that a material having a high light absorption is disposed on the back face of the liquid crystal display device.
According to the invention, the light incident from the opposite substrate portion side of the liquid crystal display device is not reflected by the signal lines but is transmitted through the insulating film and the substrate of the active matrix substrate portion to the back face of the liquid crystal display device and strikes upon the material having a high light absorption. Therefore, the light is not reflected again toward the liquid crystal layer side. Consequently, light approach to the switching element can be prevented, thereby maintaining an excellent black level.
In the invention it is preferable that the light transmitting portion is provided in the region opposite to the signal line in the opposite substrate portion.
According to the invention, in the liquid crystal display device, also in the case where a light emitting portion is provided in a region opposite to the signal line, the signal line is formed of a transparent conductive material. Therefore, light reflection on the surface of the signal line is not caused when any pixel of the liquid crystal display device performs black display. Consequently, a black level can fully be improved and a contrast can be enhanced.
In the invention it is preferable that the orientation state of a liquid crystal molecule in the liquid crystal layer is set such that the liquid crystal display device becomes normally white.
According to the invention, also in the case where the liquid crystal display device has a structure in which a color filter having a BM-less structure is formed in an opposite substrate portion, a liquid crystal in a normally while mode is used and a signal line is formed of a transparent conductive material, it is possible to prevent a contrast from being deteriorated by light reflection in the signal line.
In the invention it is preferable that the scanning lines are provided in regions between the pixel electrodes in the active matrix substrate portion,
the light transmitting portion is provided in regions opposite to the signal line and the scanning line in the opposite substrate portion, and
the scanning line is formed of the transparent conductive material.
According to the invention, in the case where the light transmitting portion is provided in the regions opposite to both the signal line and the scanning line in the liquid crystal display device, not only the signal line but also the scanning line is formed of a transparent conductive material. Therefore, when any of pixels of the liquid crystal display device performs black display, the light reflection is not caused on the surfaces of the signal line and the scanning line. Consequently, the black level can further be improved and the contrast can be enhanced still more.
In the invention it is preferable that the orientation state of the liquid crystal molecule in the liquid crystal is set such that the liquid crystal display device becomes normally black.
According to the invention, also in the case where the liquid crystal display device has a structure in which a color filter having a BM-less structure is formed on an opposite substrate, a liquid crystal in a normally black mode is used and a signal line and a scanning line are formed of a transparent conductive material, it is possible to prevent a contrast from being deteriorated by light reflection with the signal line and the scanning line.
The invention provides a liquid crystal display device comprising:
first and second insulating substrates;
a liquid crystal layer between the first and second substrates;
a plurality of pixel electrodes mutually provided on the first substrate at regular intervals;
a plurality of lines provided between the pixel electrodes;
a common electrode provided on the second substrate and opposed to two or more pixel electrodes; and
a color filter provided on the second substrate,
wherein the color filter includes color layers provided in regions opposite to the pixel electrodes and a light transmitting portion provided in regions between the color layers,
more components of light can be transmitted through the light transmitting portion than components of light which can be transmitted through the color layers, and
the line is formed of a transparent electrode material.
According to the invention, the liquid crystal display device comprises a color filter having a BM-less structure and lines disposed between pixel electrodes, comprising a transparent conductive material. For example, in the case where the liquid crystal display device has a 3-terminal element, at least one of a signal line and a scanning line are connected to the 3-terminal element. In the case where the liquid crystal display device has a 2-terminal element, the line is connected to the 2-terminal element. In the case where the liquid crystal display device has an additional capacitance, the line is connected to the additional capacitance. Thus, in the case where the line between the pixel electrodes has light transmitting properties, the light reflection is reduced on the surface of the line. Therefore, the black level of the liquid crystal display device can be improved and a contrast can be enhanced.